The Cell Cycle & Differentiation

The Cell Cycle & DNA Replication

The cell cycle consists of a period of cell growth and DNA replication called interphase, and then a period of mitosis.

DNA helicase breaks the hydrogen bonds between the two polynucleotide strands. The helix unzips to form 2 single strands.

Each original single strand acts as a template. Free DNA nucleotides join to the exposed bases.

New nucleotides are joined together by DNA polymerase. Hydrogen bonds form between bases on the original and new strand.

Each new DNA molecule contains one strand from the original DNA molecule, and one new strand.

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Mitosis has five division stages:
Interphase:
- Cell's DNA is unravelled and replicated to double it's genetic content.
- The organelles are also replicated.
Prophase:
- Chromosomes condense. Tiny protein bundles called centrioles start moving to opposite ends of the cell.
- They form a network of protein fibres called the spindle.
- The nuclear envelope breaks down and the chromosomes lie free in the cytoplasm.
Metaphase:
- Chromosomes line up along the middle of the cell and become attached to the spindle by their centromere.
Anaphase:
- The centromeres divide, separating each pair of chromatids.
- The spindles contract, pulling the chromatids to the opposite ends.
Telophase:
- Chromatids reach the opposite poles on the spindle and uncoil, forming chromosomes.
- The nuclear envelope forms around each group of chromosomes, so there's two nuclei.
- The cytoplasm divides to form two genetically identical daughter cells.

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Multicellular organisms are specialised. The structure of each specialised cell is adapted.
The process of becoming specialised is called differentiation.

Palisade Mesophyll Cells:
- Contain many chloroplasts, so they can absorb lots of sunlight.
- The walls are thin, so CO2 can easily enter.
Similar cells are organised into tissues, for example:
- Squamous epithelium tissues.
- Phloem Tissue.
- Xylem Tissue.
Tissues Are Organised Into Organs - e.g. The Leaf:
- Lower Epidermis: Contains stomata for gas exchange, it lets air in and out.
- Spongy Mesophyll: Full of spores to let gas circulate.
- Palisade Mesophyll: Photosynthesis occurs here.
- Xylem: Carries water to the leaf.
- Phloem: Carries sugars away from the leaf.
- Upper Epidermis: Covered in a waterproof waxy cuticle to reduce water loss.

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